Earnings Per Share (Schedule Of Earnings (Loss) Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Income (loss) of basic earnings (loss) per common share	$ 89,155		$ 3,705		$ 9,059		$ 15,929		$ 1,683		$ (24,022)		$ (72,136)		$ (41,149)		$ 117,848	$ (135,624)	$ (1,037,361)
Income (loss) of effect of dilutive stock options, restricted stock, and SARs																	0	0	0
Income (loss) of diluted earnings (loss) per common share																	$ 117,848	$ (135,624)	$ (1,037,361)
Weighted shares of basic earnings (loss) per common share																	51,113	50,029	49,110
Weighted shares of effect of dilutive stock options, restricted stock, and SARs																	635	0	0
Weighted shares of diluted earnings (loss) per common share																	51,748	50,029	49,110
Per share amount of basic earnings (loss) per common share	$ 1.74		$ 0.07		$ 0.18		$ 0.32		$ 0.03	[1]	$ (0.48)	[1]	$ (1.44)	[1]	$ (0.83)	[1]	$ 2.31	$ (2.71)	$ (21.12)
Per share amount of effect of dilutive stock options, restricted stock, and SARs																	(0.03)	0.00	0.00
Per share amount of diluted earnings (loss) per common share	$ 1.71	[1]	$ 0.07	[1]	$ 0.17	[1]	$ 0.31	[1]	$ 0.03	[1]	$ (0.48)	[1]	$ (1.44)	[1]	$ (0.83)	[1]	$ 2.28	$ (2.71)	$ (21.12)

[1] The earnings (loss) per share for the year's four quarters does not equal annual income (loss) per share.